All the aforementioned points, plus the Centris 650 is frequently less than a IIsi, price-wise. Then, there is the dedicated VRAM for the video circuitry, and the ability to run a 640x480 monitor at 16-bit color, or a 21" monitor at 8-bit color. The 650s with an FPU are the ones with 8MB of RAM and the AAUI port for ethernet. The 4MB Centris and Quadra 650 machines use the 68LC040 processor.
Specs on the Centris 650:
https://everymac.com/systems/apple/mac_centris/specs/mac_centris_650.html
For the Quadra 650, the differences are a 33MHz '040, and a manual inject floppy drive, instead of the older auto-inject drive.
Personally, I wouldn't go for a IIsi. It really isn't much better than an LCII, except for the fact that, yes, it can go to 65MB, and, with an adapter, can use either SE/30 PDS cards, or a Nubus card. It then has the inferior VampireVideo, whereas, at least the LC series has VRAM.
If you must have a pizza box Mac, get a Centris or Quadra 610. Still kinda limited, expansion card-wise, but, at least has an '040 chip, and the ability to use a built-in CD-ROM drive.